The present invention relates to a demodulation circuit for demodulating phase-modulated input signals and more particularly to a demodulation circuit that is suitably realized by semiconductor integrated circuits and is effectively applied, for example, to portable communication terminal apparatuses.
A circuit for demodulating phase-modulated signals may use a phase detection circuit and a binary phase detection circuit. A combination of the phase detection circuit and the binary phase detection circuit is used where a multi-value phase demodulation is required. Among literatures concerning the phase detection circuit and the binary phase detection circuit may be cited a book "Digital Modulation/Demodulation Technology for Mobile Communications" (edited by Yoshihiko Akaishi), page 137, published by Kabushiki Kaisha Trikepps on Mar. 14, 1990.
FIG. 6 shows the working principle of a demodulation circuit that the inventors studied based on the above-mentioned literature before the present invention was accomplished. A phase-modulated input signal VIN and an output VOS of a local oscillation circuit 5 are entered into a phase detection circuit 2 and a binary phase detection circuit 3. The phase detection circuit 2 converts the absolute value of a phase difference between the input signal VIN and the output VOS of the local oscillation circuit 5 into a voltage VA, which is then output. The binary phase detection circuit 3 detects a lead or lag in phase angle between the input signal VIN and the local oscillator output VOS and produces an output SIGN representing the phase lead or lag. The absolute value VA of the phase difference and the signal SIGN representing the phase lead or lag are multiplied to form a phase difference quantity VP. A difference is taken between the phase difference quantity VP and a signal VD--which is obtained by delaying the phase difference quantity VP one unit of modulation or one symbol (1 baud rate) by a delay means D--to produce a demodulated output VOUT. The delay corresponding to one symbol may be given prior to the phase detection circuit 2.